A great many forms and variations of exercise equipment are known in the prior art. Many such devices are dedicated types of exercise and training equipment; that is, equipment specifically devised to develop certain muscles or muscle groups. For example, the following patents are concerned, at least to some degree, with the exercise and development of leg muscles: U.S. Pat. No. 4,340,214, issued Jul. 20, 1982, U.S. Pat. No. 4,906,192, issued Mar. 6, 1990, U.S. Pat. No. 3,834,693, issued Sep. 10, 1974, U.S. Pat. No. 3,912,260, issued Oct. 14, 1975, U.S. Pat. No. 4,781,372, issued Nov. 1, 1988, U.S. Pat. No. 4,542,898, issued Sep. 24, 1985, U.S. Pat. No. 3,756,595, issued Sep. 4, 1973, U.S. Pat. No. 3,559,986, issued Feb. 2, 1971, U.S. Pat. No. 4,132,404, issued Jan. 2, 1979, and U.S. Pat. No. 2,969,060, issued Jan. 24, 1961.
A number of the devices disclosed in the above-identified prior art patents are characterized by their relative complexity and high expense. Furthermore, a number of such mechanisms are not intended for a general workout of all the muscles of the lower extremities but instead are devoted to muscles or muscle groups employed when carrying out certain specified activities such as skiing or skating. Also inherent in some of the prior art constructions is the fact that the user is placed, at least on occasion, in positions which are inherently unstable. This can cause falls and consequent injuries.
U.S. Pat. No. 4,340,214 is worthy of special comment. The training apparatus disclosed in such patent is for skaters and consists of a fixed training stand with two carriages transversely displaceable in opposite directions. Each carriage has a platform for attachment to one of the skater's feet and the platforms alter their angle of inclination upon displacement of the associating carriage from the initial position to provide some semblance of the action which occurs when a skate cuts the ice.
Lateral resistance to movement of the user's feet is provided through the combination of a straight inclined ramp and spring elements which are stretched as the user's foot moves up the ramp. Foot movement is rather limited in that each foot can only traverse a path extending outwardly from a midpoint location. Obviously, with such an arrangement, the user's body is continuously unbalanced and thus unstable. In fact, the device of U.S. Pat. No. 4,340,214 utilizes side platforms engaged by the shoulder of the user.
U.S. Pat. No. 3,834,693 illustrates a ski instruction apparatus featuring two independently movable platforms disposed on rails attached to a turntable. U.S. Pat. No. 3,559,986 discloses rollably mounted dollies for leg exercises. The dollies are independently movable in straight horizontally disposed pathways.